74 Dart sport 5.9 Magnum swap
Use an ECM from a 5.9 Ram 5 speed truck without security from Solo Electronics in Florida and save yourself a lot of headaches. You can reflash a performance tune with an SCT Tuner from Sean at Hemifever for impressive performance gains.
(Added) I used a 1998 ECM, bigger Hughes roller cam, a home ported Keg manifold, Summit Headers and 4-hole 25lbs Mustang injectors, along with a Hotwire Auto - Hotrod Harness. I put all this on a junkyard 5.9 that turned out to have a bad ring set in the #1 cylinder. The car was still amazingly fast with a stock stall converter & 727 with 3.23's out back.
Later I built a 408" ported Eddie headed stroker with an Airgap, Big gulp TB and larger injectors for the car and put a 200R4 overdrive behind it with 3.91's. I just reflashed the ECU and it runs great. It's truly plug and play.